Within present day cellular telephone systems, there exist audio processing circuits for processing both forward and reverse voice signals. Forward voice signals are voice signals which are carried to a land-based cell site via a telephone trunk line for transmission by a transmitter to a mobile telephone unit. Forward voice signals also include the voice signals previously received at the land-based cell site from a mobile unit for re-transmission to another mobile unit. Reverse voice signals are voice signals which are transmitted from the mobile unit to a receiver at the cell site.
Presently, forward voice signals are processed by a transmit audio processor which operates to syllabically compress and pre-emphasize the forward voice signal in order to control the level of noise in the FM environment. The transmit audio processor also serves to superimpose a supervisory audio tone (SAT), supplied from a separate signal generator, onto the forward voice signal. It is the combination of the forward voice signal and the SAT which is transmitted from the cell site to the mobile unit.
The mobile unit will transpond (send back) the received SAT signal. As long as a SAT is received at the land-based cell site from the mobile unit, the mobile unit is deemed to be in communication with the cell site for purposes of placing the telephone trunk line in an off-hook state. For this reason, a SAT detector is coupled to the receive audio processor for detecting the presence of the SAT signal transponded from a mobile unit. In addition to receiving the SAT from the mobile unit, the receiver also receives reverse voice signals which are processed by a receive audio processor which expands and de-emphasizes the received signal. For a more complete description of cellular telephone systems, and the transmit and receive voice processors contained therein, reference should be had to the Bell System Technical Journal, Jan. 1979, Vol. 58, No. 1, at pages 97-122 and 153-200, herein incorporated by reference.
At the present time, the transmit and receive audio processors are each comprised of separate analog circuits. The SAT generator and SAT detector are also comprised of separate analog circuits. A land-based cell site typically has several transmitter and receiver channels, each requiring a separate transmit audio processor and receive audio processor, respectively. Further, three separate SAT generators and detectors are required for each of the three SAT frequencies (5970,6000 and 6030 Hz) presently used in cellular telephone systems. Depending on the number of receiver and transmitter channels, the physical space consumed by the transmit and receive voice processor and the SAT detectors can be large, thus increasing the overall space required for each land-based cell site.
Thus, there is a need for a more compact apparatus for processing the forward and reverse voice signals, as well as for generating and detecting SAT signals.